The present invention relates to a film thickness-measuring apparatus which is fit for on-line measuring of the thickness of an oil film coated on the surface of a steel plate being processed in, for example, a rolling line or a plating line, and which can measure the thickness of the oil film without contacting the oil film.
The ellipsometry method is generally known as a method of measuring the thickness of a light-transmitting film such as an oil film coated on the surface of a steel plate. This method is based on the fact that the difference in state of polarization between the light reflected from the substrate steel plate with oil film coating and the light reflected from the steel plate without any coating is closely related to the thickness of the oil film. More specifically, thickness d of the oil film can be calculated from the ratio .rho. of the Fresnel complex amplitude reflection coefficient (r.sub.p) of the p component of the reflected light, which is parallel to the plane of incidence, to the Fresnel complex amplitude reflect on coefficient (r.sub.s) of the s component of the reflected light, which is perpendicular to the plane of incidence. Ratio .rho. (hereinafter called "reflection coefficient ratio") has the following relation with ellipsometric parameters .psi. and .DELTA. which serve to determine thickness d: EQU .rho.=r.sub.p /r.sub.s =tan.psi.exp.sup.j.DELTA. ( 1)
Ellipsometric parameters .psi. and .DELTA., thickness d, refractive index N.sub.1 of the oil film, refractive index N.sub.2 of the steel plate, incidence angle .phi..sub.0 of the light applied to the surface of the oil film, and wavelength .lambda. of the applied light have a relationship which can be given as: EQU .rho.=F (d, N.sub.1, N.sub.2, .phi..sub.0, .lambda.) (2)
Hence, thickness d can easily be calculated from parameters .psi. and .DELTA..
Some methods of obtaining ellipsometric parameters .psi. and .DELTA. are known. One of them is to apply elliptically polarized light generated through a compensator and a polarizer to the surface of a film whose thickness need be measured, and detect the light reflected from the layer after the light is passed through an analyzer. The analyzer and polarizer are rotated in the plane at right angles to the axis of the reflected light beam, until the analyzer passes no light at all. The angles through which the analyzer and polarizer have been rotated are then measured. Finally, ellipsometric parameters .psi. and .DELTA. are obtained from the polarizing angle and the analyzing angle.
A method of obtaining ellipsometric parameter .DELTA. is known. In this method, an analyzer of the same type as described above is rotated at high speed by an electric motor, the amount of the light passing through the analyzer is modulated with the number of rotations per unit time of the analyzer, and a photoelectric output is thereby produced. The photoelectric output, thus produced, has a component which represents ellipsometric parameter .DELTA.. Hence, parameter .DELTA. can be obtained from the photoelectric output.
The first-mentioned method has a drawback, however. It takes a few seconds to adjust the orientations of the polarizer and the analyzer so that no light is transmitted through them. Therefore, this method cannot accurately measure the thickness of the layer in the case where the film is moving fast.
The second-mentioned method is also disadvantageous. The ellipsometric parameter, .DELTA., depends to a significant extent on the reflection coefficient of the film, the amount of the light emitted from a light source, and some other similar factors. Hence, parameter .DELTA. obtained by this method cannot be so reliable. Furthermore, this method can provide only parameter .DELTA., which is just one of the two ellipsometric parameters. Therefore, the method cannot be applied to other uses than thickness-measuring, such as measuring of the refractive index of the film.